The invention relates to a combustion installation for burning a fuel, which installation includes a hearth operating as a circulating fluidized bed, and in which installation at least a fraction of the flow of solid particles resulting from the combustion of the fuel in the hearth is returned to the hearth via a heat exchanger operating as a fluidized bed.
In a circulating fluidized bed installation, the fuel, which may be crushed coal, is usually injected into the bottom portion of the hearth, where a reducing atmosphere prevails.
In that zone, the fuel is subjected to pyrolysis, with the combustible matter being separated firstly into volatile matter containing a fraction of the nitrogen from the original fuel (volatile-N), and secondly into solid matter containing the remaining nitrogen from the original fuel (coke-N). The coke grains can remain for various lengths of time in the bottom portion of the hearth, where a reducing atmosphere prevails. Under those conditions, the reaction products of the coke-N contain molecular nitrogen rather than the pollutant NOx. The time for which the coke grains remain in the reducing zone depends essentially on the size of each grain. The finest particles leave the zone rapidly and then pass through the top portion of the hearth, where the reactions continue.
More particularly in FIG. 1, the flow of solid particles coming from the hearth 1 and collected by a suitable device such as a cyclone 3 is made up not only of inorganic ash but also of coke resulting from the fuel burning incompletely in the hearth. That coke contains the original elements of the fuel, and in particular carbon, sulfur, and nitrogen.
Those solid particles are sent back to the hearth via a suitable device such as a recycling loop 4 equipped with a siphon 5. In FIG. 1, a fraction of the flow of solid particles recycled to the hearth 1 goes through a heat exchanger 7 operating as a fluidized bed. In FIG. 2, a heat exchanger 7 operating as a fluidized bed is constituted by a casing 71 containing packets of zigzag tubes 72 through which water or steam flows. The flow of solid particles enters via an inlet duct 73 and passes through the heat exchanger by means of a fluidization system and then exits via an outlet duct 74. The top level of the bed of solid particles in the heat exchanger is indicated by the line L. In this example, the heat exchanger 7 is outside the hearth 1 and only a fraction of the flow of solid particles being recycled to the hearth 1 passes through it. It is to be understood that the following description also applies when the heat exchanger 7 is part of the hearth 1 and receives the entire flow of solid particles.
In FIG. 2, the inside of the casing 71 of the heat exchanger 7 is subdivided into a plurality of chambers 75A, 75B, 75C, 75D, separated by walls 76. The chambers 75B and 75C receive the packets of tubes 72B, 72C through which water or steam passes internally. The floor 77 of the heat exchanger is equipped with nozzles 78 making it possible to inject gas for fluidizing the solids. Below the floor 77, a wind box 79 is placed that contains the fluidization gas. The wind box 79 may optionally be subdivided into at least as many compartments 79A, 79B, 79C, 79D as there are chambers inside the casing.
After passing through the solids from the bottom at floor level to above the top level L of the bed of solid particles, the fluidization gases are removed via the outlet duct 74 to the hearth 1. As the solids pass through the heat exchanger 7, their temperature is lowered from the temperature they have at the outlet of the cyclone (about 850xc2x0 C. to 900xc2x0 C.) to a temperature of about 500xc2x0 C. to 700xc2x0 C. depending on the number and the surface area of the tubes 72 inside the casing 71.
In the state of the art, the fluidization gas used in the heat exchanger 7 is atmospheric air that is usually at a temperature of in the range 20xc2x0 C. to 300xc2x0 C. and at a compression level that is sufficient to enable the solids to flow from the inlet chamber 75A to the outlet chamber 75D, for example. An atmosphere that is strongly oxidizing, or, in other words, an atmosphere that has a very high oxygen partial pressure, prevails in each chamber. That technique suffers from the drawback that nitrogen oxides form by reaction between the above-mentioned coke-N and the fluidization air of the heat exchanger 7. The production of nitrogen oxide is much higher in the inlet chambers 75A, 75B than in the outlet chambers 75C, 75D because of the amount of coke-N consumed in the inlet chambers and, above all, because the temperature of the solids is much higher in the inlet chambers than in the outlet chambers. By way of example, for a heat exchanger having four chambers as shown in FIG. 2, the temperature of the solids is typically about 850xc2x0 C. to 800xc2x0 C. in chamber 75A, about 650xc2x0 C. to 800xc2x0 C. in chamber 75B, and about 500xc2x0 C. to 650xc2x0 C. in chamber 75C. The nitrogen oxides that are formed in the chambers by reaction between the coke-N and the fluidization air are thus conveyed to the hearth, where they mix with the flue gases produced, thereby participating in the overall emission of pollutant.
An object of the invention is to decrease the emission of nitrogen oxides in such an installation.
To this end, the invention provides a method of decreasing nitrogen oxide emissions in a combustion installation for burning a fuel, which installation includes a hearth operating as a circulating fluidized bed, and in which installation at least a fraction of the flow of solid particles resulting from the combustion of the fuel in the hearth is returned to the hearth via a heat exchanger operating as a fluidized bed, wherein the heat exchanger is fed with a fluidization gas which is considerably poorer in oxygen than air.
The idea behind the invention is thus to control the oxygen partial pressure inside the chambers of the heat exchanger so as to minimize any formation of nitrogen oxides. The oxygen partial pressure is controlled to lie in the range 1% to 4%. The oxygen-poor fluidization gas, typically containing less than 12% (molar) of oxygen, is preferably constituted by flue gases preferably taken downstream from a dust filter for removing dust from the flue gases. Dust is thus removed from the flue gases, which dust could otherwise damage, by abrasion or clogging, the fans used to send the fluidization gas under pressure into the heat exchanger.
Each heat-exchange chamber of the heat exchanger is fed separately with the oxygen-poor fluidization gas used on its own or as mixed with air. By adjusting the proportion of air in the mixture, it is thus possible to obtain combustion of the carbon in the heat exchanger in the presence of an atmosphere having a low oxygen content, and thus with minimized nitrogen oxide emissions.
To avoid recycling too much flue gas into the heat exchanger, it is advantageous to feed the coldest chambers of the heat exchanger with air not mixed with the flue gases because nitrogen oxide formation is low in those chambers.